Preparation of maleimides

ABSTRACT

PREPARATION OF N-SUBSTITUTED MALEIMIDES BY HEATING THE CORRESPONDING N-SUBSTITUTED MALEAMIC ACIDS OR MONO-AMMONIUM MALEIC ACID SALTS IN THE PRESENCE OF STREAM TO A TEMPERATURE WITHIN THE RANGE OF FROM ABOUT 100* TO 400*C.

United States Patent 3,758,498 PREPARATION OF MALEIMIDES Peter Pfuller, Burgkirchen an der A12, and Wenzel Kuhn,

Feldkirchen, near Munich, Germany, assignors to Farbwerke Hoechst Aktiengesellschaft vormals Meister Lucius & Bruning, Frankfurt am Main, Germany No Drawing. Continuation of abandoned applications Scr. No. 718,254, Apr. 2, 1968, and Ser. No. 757,470, Sept. 4, 1968. This application Mar. 22, 1971, Ser. No. 126,990

Claims priority, application Germany, Apr. 27, 1967, F 52,260; Sept. 9, 1967, F 53,461 Int. Cl. C07d 27/18 US. Cl. 260-326.5 FM 9 Claims ABSTRACT OF THE DISCLOSURE Preparation of N-substituted maleimides by heating the corresponding N-substituted maleamic acids or mono-ammonium maleic acid salts in the presence of steam to a temperature within the range of from about 100 to 400 C.

CROSS-REFERENCE TO RELATED APPLICATIONS This application is a continuation of our two copending applications, Ser. No. 718,254, filed Apr. 2, 1968 and Ser. No. 757,470, filed Sept. 4, 1968, both now abandoned.

BACKGROUND OF THE INVENTION The present invention relates to the preparation of N- substituted maleimides from the corresponding maleamic acids and/or mono-ammonium maleates.

There have already been described a number of processes for the preparation of N-substituted maleimides from the corresponding maleamic acids. U.S. Pats. Nos. 3,018,290 and 3,018,292, for example, describe the conversion of N-substituted maleamic acids with acid anhydride or acid chloride, preferably acetic anhydride or acetic acid chloride, or with ketene with the addition of trialkyl amines, such as triethyl amine, preferably in a solvent. British patent specification No. 961,434 discloses the preparation of N-substituted maleimides from the corresponding maleamic acids by reacting the latter with a solution of sodium acetate in acetic anhydride at a temperature below 140 C., introducing the reaction mixture into ice water, extracting the reaction product with ether and, after removal of the ether, distilling it in vacuo. Belgian Pat. No. 654,847 describes the cyclization of the N-substituted maleamic acids in the presence of dehydrating agents, such as meta-, ortho-, pyroor polyphosphoric acid, phosphonic acid, phosphorous acid, phosphinic acid and organic sulfonic acids, preferably with the use of an inert organic diluent which is advantageously immiscible with water and has a boiling range of from 80 to 180 C.

All these processes have to be considered preparatory laboratory methods rather than processes that can be carried out on an industrial scale. They comprise several stages and yield, as could be ascertained on reproduction, considerable amounts of secondary products, inter alia derivatives of fumaric acid owing to isomerization, and give poor yields of N-substituted maleimides.

3,758,498 Patented Sept. 11, 1973 SUMMARY OF THE INVENTION Now we have found that N-substituted maleimides can be obtained from N-substituted maleamic acids and/or the mono-ammonium salts of N-substituted maleic acids, in a simple manner in good yields, by heating the starting material in the presence of superheated steam, to a temperature within the range of from about to 400 C., and separating the corresponding N-substituted maleimide from the hot reaction mixture by the injection of water. The thermal treatment of the maleamic acid or ammonium maleate can be carried out either Without the addition of accessory agents or in the presence of dehydrating agents having an acid reaction.

DETAILED DESCRIPTION OF THE INVENTION For carrying out said process according to one preferred embodiment, it is necessary to prepare, in a first step, the maleamic acid from maleic anhydride and amine in known manner, and to isolate and dry the maleamic acid. The maleamic acid so obtained is then converted into the N-substituted maleimide by heating in the presence of superheated steam to a temperature preferably within the range of from about 200 to 250 C. and obtaining the N-substituted maleimide from the hot reaction mixture in a highly pure state by the injection of water.

According to another preferred method of working according to the invention, the mono-ammonium salts of the maleic acids are obtained in a very simple manner by placing the maleic anhydride and the amine to be introduced in an aqueous solution. This solution can be subjected to dehydration without isolating the mono-ammonium salt. It is, of course, also possible to isolate the mono-ammonium salt and to use it in the solid or molten state for the dehydration. To this effect, the monoammonium salt of maleic acid, it necessary with the addition of dehydrating agents, is metered into the reaction vessel and heated in the presence of superheated steam, to a temperature preferably within the range of from to 250 C., and the formed maleimide is sep arated from the hot reaction mixture by the injection of water.

In either process the stream of steam passed into the reaction vessel continuously entrains the maleimide that has formed. In the associated cooling device the accruing minor amounts of impurities are removed by the water introduced through a nozzle.

There is preferably used superheated steam in an amount within the range of one to ten times the amount of the maleamic acid or mono-ammonium salt of the maleic acid applied. The maleimides so obtained are already in a purity above 99%, so that they can be used without further purification for polymerization reactions.

Exemplary of the dehydrating agents that may be used if desired are organic sulfonic acids, sulfuric acid, 0rtho-, diand polyphosphoric acids as well as the acid salts of polybasic acids, acid anhydrides, chlorides of zinc, magnesium and aluminum. These agents are added in an amount within the range of from 1 to 50 percent, preferably 5 to 20 percent, calculated on the maleamic acid applied, or the mono-ammonium salt.

An essential advantage of the process of the present invention is that it can optionally be carried out continuously or discontinuously. In the continuous process which gives especially good yields, the solution of mono-ammonium maleate is separately prepared, or the maleamic acid to be applied is separately melted in a container, in each instance either with or without dehydrating agents, and the melt or the solution is metered into a reaction vessel by means of a pump. Alternatively, mixtures or the maleamic acid itself are charged without pretreatment, in a pulverulent form, to the reactor by means of a screw conveyor.

As shown in the examples following hereunder, the yields attained by the process of the present invention amount to 50 percent to 90 percent of the theoretical yield, thus exceeding the yields of the prior art processes in most cases. This was an especially surprising observation when working in the presence of steam; in this case it was to be expected that owing to the steam the equilibrium of the reaction which proceeds with dehydration would be shifted, especially in the presence of large amounts of steam, quite considerably towards the starting materials.

The N-substituted maleimides obtained by the process of the present invention have a wide range of application for known industrial uses without further pretreatment or after-treatments. They may be used, for example, as monomers or 'comonomers for polymerization reactions. Moreover, they may be applied as fungicides, insecticides or f'ungistatic agents, as anti-oxidants, vulcanizers and rust preventives.

The examples following hereunder serve to illustrate the invention, but they are not intended to limit it thereto:

EXAMPLE 1 In the course of one hour, 1000 grams of molten N-cyclohexyl maleamic acid were slowly metered, in the liquid state, by means of a pump into a round-bottomed flask heated at 230 C., while simultaneously passing through superheated steam (5000 grams per hour). Into the hot reaction gases pre-cooled to approximately 150 C., water was introduced through a nozzle for condensing the steam, whereon N-cyclohexyl maleimide was obtained in a nonacid and finely crystalline form (melting point: 87 to 90 C.).

The yield amounted to 53 percent of the theoretical.

EXAMPLE 2 In a series-connected apparatus, a mixture of maleic anhydride was reacted with cyclohexyl amine to give N- cyclohexyl maleamic acid. 85 kg. of this reaction product were mixed, per hour, with 15 kg. of ortho-phosphoric acid of 85 percent strength and passed, with the admixture of 200 kg. per hour of superheated steam, through a tubular reactor at a temperature ranging from 220 to 240 C. Via an indirect cooling apparatus in which the reaction mixture was cooled to approximately 120 C., the reaction mixture travelled to another cooling chamher in which the steam was condensed by introducing water by means of a nozzle. The corresponding N-cyclohexyl maleimide simultaneously separated out in a pulverulent form so that, after filtration from the aqueous condensate, it could be used without additional purification for the copolymerization with vinyl chloride.

The N-cyclohexyl maleimide obtained in this manner melted at 87 to 90 C.; the yield amounted to 91 percent of the theoretical.

When the N-cyclohexyl maleimide was obtained by one of the known methods, the yield amounted only to 30 percent to 60 percent.

EXAMPLE 3 200 grams of para-toluene sulfonic acid were added to 800 grams of N-phenyl maleic amide and treated, as described in Example 1, at a temperature of 280 C. in the presence of eight times the amount of superheated steam. A yellow N-phenyl maleimide of a melting point of 89 C. was obtained in a yield of 82 percent of the theoretical.

4 EXAMPLE 4 750 grams of N(9,10-ethylene-9,10-dihydr0-1l-anthrylmethyD-maleamic acid were mixed with 250 grams of potassium hydrogen sulfate and charged in the solid state by means of a screw conveyor to the apparatus described in Example 1. At a temperature of 250 C., approximately 2000 grams of steam were passed in during the course of one hour.

A colorless N-endoethyleno-9,10-dihydro-1l-anthrazenomethyl maleimide having a melting point of 180 to 182 C. was obtained in a yield of 51 percent of the theoretical.

EXAMPLE 5 16 kg. of an aqueous solution of monocyclohexylammonium maleate of 62% strength, which contains 15% by weight of phosphoric acid (calculated on the amine salt), were metered in the course of one hour, while simultaneously passing in 20 kg. of steam having a temperature of 230 C., into the reaction vessel which was maintained at a temperature of 180 to 200 C. Water was introduced through a nozzle into the hot reaction gases, which had been pre-cooled to 150 C., for condensing the steam whereupon N-cyclohexyl maleimide was obtained free from acid in a pulverulent form in a yield amounting to 87% of the theoretical amount by weight. The product could be used without further afterpurification for copolymerizations.

EXAMPLE 6 1000 grams of an aqueous solution of mono-tert.butylammonium maleate of 62% strength were metered into a cyclization vessel and treated as described in Example 5. Yield of N-tert.buty1 maleimide: 47% of the theoretical amount.

EXAMPLE 7 90 parts of monoethyl-ammonium maleate were dissolved in 60 parts of water, 10 parts of concentrated H SO were added thereto, and the whole was treated in the manner as described in Example 5. N-ethyl maleimide was obtained in a yield amounting to 60% of the theoretical amount.

EXAMPLE 8 200 grams of para-toluene sulfonic acid were stirred into 1400 grams of an aqueous solution of mono-phenylammonium maleate, and the solution was treated in the manner as described in Example 5. The N-phenyl maleimide was obtained in the form of yellow crystals in a yield amounting to 75% of the theoretical yield.

EXAMPLE 9 750 grams of mono-N-(9,10-dihydro-9,IO-ethano-anthryl-(l1)-methyl)ammonium maleate were mixed with 250 grams of potassium hydrogen sulfate, the mixture was charged in the solid state to the reaction vessel by means of a screw conveyor and treated in the manner described in Example 5. N-(9',10-dihydro-9,IO-ethanoanthryl-(11)-methy1)-maleimide was obtained in a yield of 47 of the theoretical amount.

We claim:

1. In a process for the preparation of N-substituted maleimides from a correspondingly N-substituted starting material selected from the group consisting of maleamic acids and monoammonium salts of maleic acids, wherein the formed N-substituted maleimide is separated from the hot reaction mixture by the injection of water, the improvement which comprises heating the said starting material in the presence of superheated steam to a temperature within the range of from C. to 400 C.

2. The process of claim 1 wherein said starting material 18 a mono-ammonium salt of maleic acid.

3. The process of claim 1 wherein said starting material is a maleamic acid.

4. Process as defined in claim 1, in which the thermal is heated to a temperature within the range of from about treatment is carried out in the presence of about 1 to 200 to 250 C. 50% of dehydrating agent having an acid re i n. 9. The process of claim 2 wherein the said salt is heated 5, h process f claim 1, in which the superheated to a temperature within the range of from about 150 C. steam is applied in an amount within the range of one to 5 to ten times the amount of the starting material used. References Cited 6. The process of claim 1, in which the starting material UNITED STATES PATENTS is charged to the reactor in the molten state. 3,018,290 1/1962 Sauers et a1 7. The process of claim 1, in which the process is 10 3 394 145 7 19 Bublitz 26O 3265 F M carried out in a continuous manner.

8. The process of claim 3 wherein the maleamic acid JOSEPH A NARCAVAGE, Primary Examiner 

